Microphones are ubiquitous on many devices used by individuals, including computers, tablets, smart phones, and many other consumer devices. Generally speaking, a microphone is an electroacoustic transducer that produces an electrical signal in response to deflection of a portion (e.g., a membrane or other structure) of a microphone caused by sound incident upon the microphone. For example, a microphone may be implemented as a MEMS transducer. A MEMS transducer may include a diaphragm or membrane having an electrical capacitance, such that a change in acoustic pressure applied to the MEMS transducer causes a deflection or other movement of the membrane, and thus causes a change in the electrical capacitance. Such change may be sensed by a sensing circuit and processed.
Sensing of a MEMS transducer may rely on a constant charge present on the electrical capacitance of the MEMS transducer. Thus, a large bias voltage, typically higher than a breakdown voltage of the MEMS transducer, may be used to bias the MEMS transducer. Therefore, it is often necessary to protect a MEMS transducer to prevent too large of a voltage appearing on the electrical capacitance of the MEMS transducer. Such protection is often achieved with a voltage clamp, which may be implemented with diodes. However, when such a voltage clamp activates during a very large input (e.g., very high acoustic pressure), charge may be added or removed from the electrical capacitance of the MEMS transducer. When the large input is removed, the charge on the electrical capacitance must recover from the point at which the voltage clamps to its original charge. This voltage recovery can cause a large voltage offset of the microphone, which may cause audio artifacts (e.g., clipping, distortion) that last several seconds until the charge on the electrical capacitance returns to its original level. Such a large input event may be referred to as a “plosive event.” A plosive event may be defined as any event in which the MEMS transducer is exposed to an input (e.g., very high acoustic pressure) greater than a peak input, such that undesirable charge is added or removed from the electrical capacitance of the MEMS transducer. A plosive event may include a “pull-in event,” in which capacitive plates of the electrical capacitance of the MEMS transducer electrically short together (e.g., due to very high acoustic pressure). Such plosive events may cause high-impedance nodes of sensing circuitry coupled to the MEMS transducer to lose charge, leading to reduced sensitivity of the MEMS transducer, and in some cases, loss of functionality due to signal clipping or other audio artifacts.
Traditional analog MEMS transducers typically rely on diodes to prevent overload and to provide a low-impedance path to replenish charge in a MEMS transducer responsive to a plosive event. However, such approach may be disadvantageous as it may require design tradeoffs. If too few diodes are used, the diode conduction path may turn on at expected large audio inputs, causing distortion of audio signals. On the other hand, if too many diodes are used, then they may not return high-impedance nodes of a sensing circuit to a value close enough to its direct current level to settle back to normal operation in a reasonable amount of time.